This thesis presents a new approach to the long-standing question of how electoral rules influence the behaviour of legislators. It begins with the argument that fresh empirical advances can be made by moving beyond the pervasive but rigid assumption that all legislators want to be re-elected and, by extension, that every incumbent values this goal to the same degree. Rather, I propose that individual Members of Parliament (MPs) vary in the extent to which they personally desire or depend upon re-election. Following the principles of a difference-in-differences design, this observation allows me to devise a theoretical framework capable of testing whether MPs' vote-seeking behaviour differs within parliaments in a way that varies predictably across countries. Specifically, I propose that in electoral systems where party-centric behaviour increases re-election chances, MPs particularly invested in the goal of re-election should cater to the party to an even greater extent than their colleagues. Conversely, in systems where a personal vote can generate electoral gains, MPs most ambitious for re-election should engage in this type of vote-winning strategy to the greatest extent. I test this prediction across the UK House of Commons and the German Bundestag, and within Germany's mixed-member system. Newly-collected biographical data on over 1700 MPs is used to conduct the first systematic MP-level operationalisation of re-election ambition based on legislators' career backgrounds. Career politicians are thereby identified as those most ambitious for re-election. Using voting behaviour from 1.8 million vote choices in legislative roll-calls as a proxy for the degree to which an MP caters to the party or to his or her personal reputation, the quantitative multilevel analysis reveals strong evidence for the proposed behavioural pattern. The contribution made by this study is two-fold. First, it uncovers the interaction between electoral rules and individual re-election ambition as a new explanation for MP-level variation in legislative behaviour. Second, its research design overcomes shortcomings in previous empirical tests for the existing theory on how electoral rules impact MP behaviour (e.g. Carey and Shugart 1995), producing more robust evidence in support of this influential framework.